hung line help needed

[Chris Torek]

Actually, there *is* a bug in the 4.2BSD sleep; it's just that the bug
isn't a missing sigsetmask() but two missing calls (sigblock() and
sigsetmask()).  It is *always* *always* *always* a mistake to call
sigpause() to await a signal when the new mask is trying to unblock a
signal that has never been blocked.  (Often it works, but it is still a
mistake.)  The reason is simple: if it's not blocked now, the signal
might happen between the C ``sigpause()'' call and the actual entry
into system code.  System calls are not atomic operations (at user
level) until kernel code is entered; that's *why* sigpause is *defined*
as ``atomically set signal mask and await signal''.

(Gee, maybe we should kludge up the kernel to gripe about sigpause()s
that don't release any signals, giving the name of the offending
program... :-) )
-- 
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In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (301) 454-7690
UUCP:	{seismo,allegra,brl-bmd}!umcp-cs!chris
CSNet:	chris at umcp-cs		ARPA:	chris at maryland